Some are, some aren't. A normal hatchling would cost about $15 to $20 (US dollars). That's about the same amount of money as a music CD. There are dozens of different genetic morphs though, and they all cost different amounts. Hatchlings of the more popular ones are more like $100.
A decade ago they were a LOT more expensive, or so I hear. The craze with corn snakes has died down a lot. Right now the snake-of-choice for breeders in America is the ball python. And they are holding the prices artificially high. It's really strange. You can get a normal ball python for $30 almost anywhere. But anything other than a normal and you're looking at shelling out thousands of dollars. I've seen some upards of $20,000. Rediculous.